SRT subtitles work for me with ffdshow tryouts DXVA decoder by using the decoder's built-in subtitle support.
For instance, MKV, AVC, DTS, embedded SRT inside MKV, ATI Radeon HD 4200:
Filter Graph Info:
Filter 'JRiver Audio Renderer'
CLSID: {A4002F8E-510F-442C-8AD3-F9C7B23FB394}
Host:
Input Pin 'In'
Connected to pin 'Out' of filter 'ffdshow Audio Decoder'
Major type MEDIATYPE_Audio Sub type MEDIASUBTYPE_IEEE_FLOAT, Format type FORMAT_WaveFormatEx
Filter 'Video Mixing Renderer 9'
CLSID: {51B4ABF3-748F-4E3B-A276-C828330E926A}
Host: c:\windows\system32\quartz.dll
Input Pin 'VMR Input0'
Connected to pin 'Out' of filter 'ffdshow DXVA Video Decoder'
Major type MEDIATYPE_Video Sub type Unknown GUID Name: {1B81BE68-A0C7-11D3-B984-00C04F2E73C5}, Format type FORMAT_VideoInfo2
Input Pin 'VMR Input1'
Filter 'ffdshow Audio Decoder'
CLSID: {0F40E1E5-4F79-4988-B1A9-CC98794E6B55}
Host: c:\program files\combined community codec pack\filters\ffdshow\ffdshow.ax
Output Pin 'Out'
Connected to pin 'In' of filter 'JRiver Audio Renderer'
Major type MEDIATYPE_Audio Sub type MEDIASUBTYPE_IEEE_FLOAT, Format type FORMAT_WaveFormatEx
Input Pin 'In'
Connected to pin 'Audio' of filter 'Haali Media Splitter'
Major type MEDIATYPE_Audio Sub type Unknown GUID Name: {00002001-0000-0010-8000-00AA00389B71}, Format type FORMAT_WaveFormatEx
Input Pin 'In2'
Filter 'ffdshow DXVA Video Decoder'
CLSID: {0B0EFF97-C750-462C-9488-B10E7D87F1A6}
Host: c:\program files\combined community codec pack\filters\ffdshow\ffdshow.ax
Input Pin 'In'
Connected to pin 'Video' of filter 'Haali Media Splitter'
Major type MEDIATYPE_Video Sub type Unknown GUID Name: {31435641-0000-0010-8000-00AA00389B71}, Format type FORMAT_MPEG2_VIDEO
Output Pin 'Out'
Connected to pin 'VMR Input0' of filter 'Video Mixing Renderer 9'
Major type MEDIATYPE_Video Sub type Unknown GUID Name: {1B81BE68-A0C7-11D3-B984-00C04F2E73C5}, Format type FORMAT_VideoInfo2
Input Pin 'In Text'
Connected to pin 'Subtitle' of filter 'Haali Media Splitter'
Major type Unknown GUID Name: {E487EB08-6B26-4BE9-9DD3-993434D313FD} Sub type Unknown GUID Name:
{87C0B230-03A8-4FDF-8010-B27A5848200D}, Format type Unknown GUID Name: {87C0B230-03A8-4FDF-8010-B27A5848200D}
Input Pin 'In Text 2'
Filter 'Haali Media Splitter'
CLSID: {55DA30FC-F16B-49FC-BAA5-AE59FC65F82D}
Host: c:\program files\directshow filterpack\haali splitter\matroskasplitter\splitter.ax
Output Pin 'Video'
Connected to pin 'In' of filter 'ffdshow DXVA Video Decoder'
Major type MEDIATYPE_Video Sub type Unknown GUID Name: {31435641-0000-0010-8000-00AA00389B71}, Format type FORMAT_MPEG2_VIDEO
Output Pin 'Audio'
Connected to pin 'In' of filter 'ffdshow Audio Decoder'
Major type MEDIATYPE_Audio Sub type Unknown GUID Name: {00002001-0000-0010-8000-00AA00389B71}, Format type FORMAT_WaveFormatEx
Output Pin 'Subtitle'
Connected to pin 'In Text' of filter 'ffdshow DXVA Video Decoder'
Major type Unknown GUID Name: {E487EB08-6B26-4BE9-9DD3-993434D313FD} Sub type Unknown GUID Name:
{87C0B230-03A8-4FDF-8010-B27A5848200D}, Format type Unknown GUID Name: {87C0B230-03A8-4FDF-8010-B27A5848200D}
Not all tryouts builds are good. The above graph is from ffdshow tryouts rev. 3825. I just tried the rev. 3851, but DXVA decoding didn't work at all. Playback didn't advance. I installed the rev. 3825 back and the problem disappeared.